Not quality of output. Not speed of output - as long as that's above a minimum.
But marketing effectiveness. Which is closely related to marketing spend.
DOOM was dropped into an ecosystem of PC magazines and BBSs and almost sold itself - literally with the shareware release. It had almost no obvious competitors. There was some ad spend, but not a huge amount.
Today there are thousands and thousands and thousands and thousands of small/medium games available across multiple platforms. Even for an exceptional game, getting traction in the market is far harder than it used to be.
Meanwhile the AAAs have a budget for carpet bomb marketing. Skyrim spent around $15m, which is an insane sum.